Early Devonian (Emsian) Deposits in the Bolshaya Usa Section (Polar Urals): Lithology, Stratigraphy, and Isotopic Geochronology

Abstract This article presents the results of a comprehensive lithological, biostratigraphic, and isotope geochronological study of the Emsian Bolshaya Usa River section on the western slope of the Polar Urals. The study aims to characterize the boundary between the Vuchvozh and Shervozh formations, and to clarify their stratigraphic range, to reconstruct the depositional settings, and to identify the sources of clastics for the formations studied. The detailed bed-by-bed description and analysis of the lithological and paleontological data enabled us to perform a lithological and stratigraphic subdivision of the section. The boundary between the Vuchvozh and Shervozh formations is marked by the appearance of a bed of fine-grained quartz sandstone and calcareous argillite. This siliciclastic unit has been proposed as a marker horizon. The deposition likely occurred in the lagoonal part of a carbonate platform in a back-reef setting. Biostratigraphic analysis of conodont, stromatoporoid, and tabulate associations from the Vuchvozh Formation indicates an Emsian age for the rocks, corresponding to the upper part of the Vyazovaya, Koyva, and the lower part of the Biya regional stages of the regional stratigraphic scheme of the western slope of the Urals (approximately corresponding to the serotinus–patulus conodont zones). U–Pb dating of detrital zircons from sandstones at the base of the Shervozh Formation revealed the predominance of Meso- and Paleoproterozoic ages (1–2 Ga), with a subordinate amount of Neoarchean-Paleoproterozoic and Neoproterozoic zircons. The distribution of detrital zircons indicates that the main sources of detrital material delivered to the passive Arct-Laurussia margin during the Emsian were the formations of the Sveconorwegian and Svecofennian domains of the Baltica, exposed as a result of the Caledonian orogeny. The dating results are consistent with existing paleogeographic models of the region.